In the context of fossil-fired power plants for generating electrical energy on a large scale, a waste gas containing carbon dioxide is produced as a result of the combustion of a fossil fuel. In addition to carbon dioxide, the waste gas contains further combustion products such as the gases nitrogen, sulfur dioxide, nitrogen oxide and water vapor, for example, as well as solid particles, dusts and soot. After largely separating from the solid-particle components, the waste gas is released into the atmosphere; the carbon dioxide collecting in the atmosphere prevents the radiation of heat from our planet and contributes to an increase in the surface temperature of the earth due to the so-called greenhouse effect.
In order to achieve a reduction in the carbon dioxide emissions in the context of fossil-fired power plants, carbon dioxide can be separated from the waste gas.
Various methods for separating out carbon dioxide from a gas mixture are known, in particular from the chemical industry. In particular, the “absorption-desorption” or low-temperature separation (“cryogenic”) method is known for separating out carbon dioxide from a waste gas following a combustion process (post-combustion CO2 separation).
On a large scale, the described separation of carbon dioxide using the absorption-desorption method is effected by means of a washing agent. In a conventional absorption-desorption process, the waste gas is brought into contact with a selective solvent as a washing agent in an absorption column, and the absorption of carbon dioxide is effected by a chemical or physical process in this case.
The carbon-dioxide-loaded solvent is carried to a desorption column for the purpose of separating out the carbon dioxide and regenerating the solvent, wherein the separation in the desorption column can take place thermally. In this case, a gas-vapor mixture of gaseous carbon dioxide and vaporized solvent is driven out of the loaded solvent. The vaporized solvent is then separated from the gaseous carbon dioxide. The carbon dioxide can now be condensed, cooled and liquefied in a plurality of stages. In liquid or frozen state, the carbon dioxide can then be supplied to a storage or recycling facility. The regenerated solvent is fed back to the absorption column, where it can absorb carbon dioxide from the carbon-dioxide-bearing waste gas again.
A central problem in the existing method for separating out carbon dioxide from a gas mixture is in particular the very high energy expenditure that is required in the form of heat energy for the desorption. In order to counter this problem, a range of proposals are known from the prior art.
In the context of gas washing in the chemical industry, use is often made of physical washing agents. In the case of CO2 separation using physical washing agents, comparatively limited capacities and selectivities of the absorbing agent must nonetheless be accepted as disadvantages. As a result of this, physical washes are only considered in the case of comparatively high partial pressures, and are therefore ruled out for the post-combustion capture processes. In the case of what are known as chemical washing agents, a significantly higher loading of the absorbing agent with carbon dioxide can be achieved due to the chemical reactions involved. In this case, the acidic gas carbon dioxide is bonded with a base. Amino compounds are examples of base reaction partners. The basicity or the base strength value (pKb value) has a crucial role in the CO2 capacity and the desorption energy in this case. The higher the pKb value, the further the reaction balance lies on the side of the resulting carbamates and bicarbonates/hydrogen carbonates. As a consequence, however, there is also a stronger bond and hence a higher reaction enthalpy that must be applied again for the desorption. Until now, it has always been necessary to accept the result of the these opposing effects as unavoidable and that therefore, in particular in the case of the reactive washing agents or solvents, a high energy input was required for the desorption process in the desorption column, this being at the expense of the overall efficiency of the power plant.